Toyota prius Owner's Manual page 583

Oil viscosity (0W-20 is explained here as an example):
• The 0W in 0W-20 indicates the characteristic of the oil which
allows cold startability. Oils with a lower value before the W allow
for easier starting of the engine in cold weather.
• The 20 in 0W-20 indicates the viscosity characteristic of the oil
when the oil is at high temperature. An oil with a higher viscosity
(one with a higher value) may be better suited if the vehicle is
operated at high speeds, or under extreme load conditions.
How to read oil container labels:
Either or both API registered marks are added to some oil containers
to help you select the oil you should use.
PRIUS_OM_OM47871E_(EE)
6-1. Specifications
API Service Symbol
Top portion: "API SERVICE SN"
means the oil quality designation
by American Petroleum Institute
(API).
Center portion: "SAE 0W-20"
means the SAE viscosity grade.
Lower portion: "Resource-Con-
serving" means that the oil has
fuel-saving and environmental
protection capabilities.
ILSAC Certification Mark
The International Lubricant Speci-
fication Advisory Committee
(ILSAC) Certification Mark is dis-
played on the front of the con-
tainer.
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